Toward a Better Understanding of Ferric-Oxalate Complex Photolysis: the Role of the Aqueous/air Interface of Droplet

In this work, the photo reactivity of ferric oxalate (Fe(III)-Ox) complex in atmospheric particles was investigated. Raman spectroscopy was used to explore the mechanism and kinetics of Fe(III)-Ox photolysis occurring at the aqueous/gas interface, inside the droplet and in bulk solution. Ferrous carbonate (FeCO3) was detected indicating that carbonate ion (CO32-) formed inside the droplets would compete with oxalate ligands for iron complexation. A higher concentration of photoproduct Fe(II)-Ox was observed at the surface and inside of the droplets than in bulk solution. In particular, Fe(III)-Ox on the droplet surface was quickly reduced with light and Fe(II)-Ox concentration gradually decreased with irradiation time. The evolution of Fe(II)-Ox concentration was similar inside the droplet and in bulk solution with a trend of first increasing and then gradually decreasing during irradiation time. Although FeCO3 would hinder Fenton intermediate reaction, the photolysis rate of Fe(III)-Ox in droplets was almost two orders of magnitude times faster than that observed during bulk experiment. In general, the photolysis mechanism and kinetics of Fe(III)-Ox in aqueous/air interface, inside of droplet and bulk solution were distinct, and the production of oxide species from the atmospheric Fe(III)-Ox droplets was underestimated.